In recent years L class vacuums have proven to be inadequate on construction sites hence the introduction and now widespread use of M Class dust extractors. The HSE insist on it. An M class vacuum may not necessarily have more suction power than an L Class extractor, but they have far more advanced filters. This is a crucial difference and means you are less likely to suffer health problems.
The HEPA (High-Efficiency Particulate Air) filters you find on most M Class extractors are considerably more effective than the filters on L Class extractors and collect 99.9% of the dust passing through them.